In wireless voice and data communications, there is a trend towards making a single communications device that is capable of operating with multiple communications protocols, such as global system for mobile telephony (GSM), enhanced GSM (EGSM), GSM digital communications system (GSM DCS), GSM personal communications system (GSM PCS), code-division multiple access cellular band (CDMA LB), CDMA PCS (CDMA HB), wideband CDMA cellular band (WCDMA LB), WCDMA PCS (WCDMA MB), WCDMA unified mobile telecommunications system (WCDMA HB), global positioning system (GPS), and so forth. These communications devices are commonly called “World Phones” since the intent is to have a single device that is compatible with the many different communications protocols in use throughout the world.
In order to be compatible with a large number of communications standards, a communications device must be compatible with the different communications protocols in use as well as be able to send and receive properly encoded information within the multitude of different operating frequency ranges that are in use throughout the world. Compatibility with the different communications protocols can be achieved with the use of adequate processing power and programming, while the ability to send and receive information in the different operating frequency ranges can require a significant amount of front-end hardware in terms of filtering, up/down conversion, amplification, and so forth. Front-end hardware can typically be defined as hardware between a device's antenna and hardware that is used for demodulating and decoding the received signal. This can be further exacerbated by the fact that certain communications protocols, such as the various CDMA and WCDMA communications protocols, require that both the transmitter and the receiver be on simultaneously. This can cause problems at the receiver since signals produced by the transmitter can overpower any signal that the receiver is attempting to detect (due to the close proximity of the transmitter to the receiver). In the case of GPS, the transmitted signal from the mobile phone with which the GPS receiver co-exists poses the problem of GPS receiver de-sensitization.
In addition to front-end circuitry for a receiver, similar circuitry is also needed at a transmitter side of the communications device. Filtering is also needed at the transmitter side to help prevent the situation wherein a signal being transmitted bleeds out of its designated frequency band and into other frequency bands, such as the frequency band of the receiver, for example.
One commonly used technique to afford the ability to send and receive information in the different operating frequency ranges is to have a different set of front-end hardware for each of the communications protocols and operating frequency ranges being supported by the communications device. This can provide the communications device with the ability to communicate using each communications protocol at any of the operating frequency range that may be used.
A technique that can be used to solve the problem of signals provided by the transmitter drowning out the receiver involves the use of a high-performance filter, such as a standing acoustic wave filter (SAW filter), that can be used to separate the received signal from the transmitted signal. The SAW filter is commonly referred to as an interstage filter. Passing the received signal through the high-performance filter can effectively isolate the received signal from the transmitted signal, thereby preventing the transmitted signal from overpowering the received signal.
One disadvantage of the prior art is that the presence of a different set of front-end hardware for each communications protocol at each operating frequency range can result in a large amount of additional front-end hardware, especially for communications devices that are compatible with a large number of communications protocols. Furthermore, a similar amount of hardware may be necessary for a transmit portion of the communications device. The large amount of hardware can be detrimental to the communications device in several ways, such as decreasing the performance, increasing the overall size and weight of the communications device, decreasing the reliability of the communications device, and so on.
A second disadvantage of the prior art technique of using the SAW filter is that the SAW filter is not conducive to integration. The inability to integrate the SAW filter requires that the signal path through the front-end hardware alternate between being on-chip (processed by circuitry that has been integrated into an integrated circuit) and off-chip (processed by circuitry that has not been integrated into an integrated circuit). When a signal makes the transition from on-chip to off-chip (or off-chip to on-chip), matching networks are needed to provide necessary impedance matching to help reduce signal loss. This can result in an increased in hardware requirement, as well as increased manufacturing costs, and decreased reliability. Furthermore, SAW filters are not tunable, a SAW filter for one band cannot be tuned to another band. Distinct SAW filters are needed for each band.
Yet another disadvantage of the prior art technique of using the SAW filter is that by going on and off chip within the signal path increases the number of input/output pins required for integrated circuits that are being used in the signal path. By increasing the number of input/output pins, the overall package size of the integrated circuits can increase as well as the overall cost of the integrated circuits, since packaging can account for a significant portion of the cost of the integrated circuit.